Interesting problem with loft condensation...
100 year old house, no water/plumbing in the loft. Loft is lined and retiled, probably 40+ years ago. Loft has always been bone dry, at least through to last summer. Loft is well sealed from the house, but there's pretty much no moisture source in the house anyway (far less than there would normally be due to lifestyle of the occupant).Last summer, various maintenance activities were carried out around the loft and roof.
Some lead flashing was replaced, and that still seems to be fine - no water penetration. (Prior to that, a small amount of rain water was running down the chimney breast in the loft, but it wasn't actually causing any problems, and loft was otherwise bone dry.) Chimney breast is now bone dry, so that's fixed. Indeed, there's no sign of rain penetration anywhere else either.
Another 4 inches of loft insulation were added, on top of the 2-3" that have been there > 25 years.
I replaced the facia and gutters along one side. There was no ventilation there before (house has no soffits), and I spaced the new facia a little bit off the wall plate timber to allow a little ventilation up between them.
Whilst there was access to the chimney, I took opportunity to add more ventilation in the form of air bricks in the gable end (one large, one smaller), as the loft wouldn't meet modern ventilation requirements, although as I said before, it had never got damp before.
Just checked it after the winter, and it's obvious that the rafters have all been very wet over winter, which has never happened before. It's spread across the whole roof, so it's condensation, not a leak. Being an old house with untreated timbers, I can't let this continue.
So I'm trying to work out the cause, and it would seem to match something I have hypothesised before, and this is what I think has happened.
Before I did the work, there was a little loft ventilation, and little loft insulation. Heat would leak from the house, warm the loft, and consequently reduce the relative humidity of the (outside) air in the loft so there was no chance of any surfaces lower than the dew point, as the loft will be warmer than the outside air.
Now, the increased ventilation means more outside air is passing through the loft. The increased insulation means the loft isn't being warmed. On cold clear nights, exposed surfaces such as a roof radiate heat and in the absence of any heat source, cool below the outside air temperature. This will now drop below the dew point of the outside air, and form dew/condensation on the inside of the roof, in the same situation you get dew outside.
Does this sound plausible?
Any bright ideas on fixing it, apart from rolling up the loft insulation and blocking off the new air vents? It seems to me such a roof is dependent on heat leaking from the house.